Voltage regulator voltage spike

[TBS]

Hi guys,

I am trying to build a voltage regulator for my project (1.jpg). I am using low dropout power regulator L4940V85 (2.jpg) with electrolytic (16V, 100uF) and ceramic (500V, 0.1uF) capacitor at the input and output.

The problem is that the project is extremely sensitive to voltage fluctuation and it must not exceed 9V. Unfortunately, when I connect the power source to the voltage regulator it produces a spike for about 30ms of much higher voltage (3.png, 4.png – Siglent SDS1202X-E). This spike is present with or without capacitors and with or without load at the output.
I also tried different buck converters to solve the problem (5.jpg), but unsuccessfully.

Any suggestions how to limit the spike?

[alm]

What is your probing setup like? Do you see the same if you probe directly at the regulator pins using a short springy ground clip around the front ground sleeve of the probe? What does the input to the regulator look like (using again a low inductance probing setup)?

[PartialDischarge]

Do you have one of the disc capacitors connected from input to output?

[TBS]

Thank you guys for replying

What is your probing setup like? ...

I added a new picture (6.jpg) for the probing setup - using only the probe of the oscilloscope connected either directly to the regulator output or to the cable (white is negative, blue is positive) gives the same results. I am using lowESR capacitors.

Do you have one of the disc capacitors connected from input to output?

Nope, I attached new diagram of the setup to the first post (7.jpg). Every capacitor is connected from positive to negative only before and after the regulator.

[StillTrying]

There may be a number of things, so here's some random ones. 

It might be a scope thing, to get an accurate capture of a rising a edge keep the initial 0V level on the scope screen, right at the very bottom would do. Often when a trace first renters the screen area a scope can do funny things to the first edge before it recovers from the slightly out of range voltage.

There might be a bad GND connection right in the middle of the breadboard causing the GND voltage to rise during the power on surge.

Where you've got the 2 crock clips, it's always a good idea to have the 2 wire lengths 15-25mm different, so that the crocks can't touch.

[alm]

I added a new picture (6.jpg) for the probing setup - using only the probe of the oscilloscope connected either directly to the regulator output or to the cable (white is negative, blue is positive) gives the same results. I am using lowESR capacitors.

The setup you show has a very high ground lead inductance. Not only does the clip lead already have a fair amount of inductance, but the extra wiring adds even more to that. Even though you would not expect this to be an issue at this low sweep speed, especially with a 10x probe, it pays to use good technique when trouble shooting. Save the sloppy technique for when everything is working .

Different test: Does the scope show anything when you leave the ground lead connected like in the picture, but clip the probe to the ground pin of the regulator instead of the output? This checks for common mode signals that the probe might pick up due to the large ground lead loop area.

I believe I already suggested probing the input to see if there are some extreme spikes there.

[MK14]

Is it a fake ST voltage regulator ?

E.g. bought cheaply on Ebay or other Chinese sources ?

The ST symbol does not look right to me (but I could be wrong), and the 5's on the right hand side, look identical, EXCEPT one of them has NOT been inked properly (the top right hand side one, hard to see).

Fake ones tend to mis-behave and/or not work properly.

Thread about fake ST regulators:
https://www.eevblog.com/forum/projects/abysmally-poor-l7805cv-performance-counterfeit-or-%27cv%27-variant-limitation/

[alm]

It would have been nice if ST would have put any limitations in the datasheet. Since there are LDOs that are stable with ceramic (and thus low ESR) output caps. But ST always seemed stingy to put more than the bare minimum in their datasheets, unlike National (before the takeover) and Linear.

[amspire]

I am trying to make sense of the scope output. It seems to start at 3.44v and then peak at 13.44v. Where is this 3.44V from?

If the scope is set up in a way that it is clipping at 3.44V and cannot get below that, it means the scope input is going from a state of overload on power on, and the scope may have some kind of overload recovery artifact if you have actually driven an input amplifier to clipping with a zero volts input plus whatever offset you have set. Choose a range that can go from 0V to 15V without any clipping and try again.

If the scope is not the source of the 3.44V, is it something to do with the nature of the power source, and perhaps an issue with the scope ground integrity? Have you tested the scope ground clip for continuity back to the scope ground? Do you have a pair of 9v batteries you can use to power the circuit instead? The 2x9V batteries would at least be a perfect floating power source so you can eliminate the power source as an issue.

blueskull is correct in that regulators are often not stable with too much low esr capacitance on the output (the datasheet seems to use 22uF caps), but I cannot see that being the cause of such a big slow spike.

[MK14]

Reduce your output capacitance and remove the ceramic cap.
This LDO is not supposed to be run with high output capacitance nor low ESR.

But the OP said:

This spike is present with or without capacitors and with or without load at the output.

So if your theory is causing the issue, removing the capacitors, should solve or change the overshoot.

BUT the OP should probably listen to your suggestions, anyway. As getting it wrong (the capacitors) can mess the circuit up, as you described.

[Andreas]

This spike is present with or without capacitors and with or without load at the output.

According to the data sheet the minimum load has to be 5 mA.
The maximum allowed input voltage is 17V.

With best regards

Andreas

[StillTrying]

I am trying to make sense of the scope output. It seems to start at 3.44v and then peak at 13.44v. Where is this 3.44V from?

I assumed the very thin horizontal line at 3.44V was just another scope thing, but now I've seen were the X trigger position is I think the pulse is just sinx's interpretation of the start of memory!

The OP will have to try to get the "switch on" nearer the center X trigger position.